Semiconductor temperature sensors are produced in the form of ICs. Their fundamental design results from the fact that semiconductor diodes have temperature-sensitive voltage vs. current characteristics.
The use of IC temperature sensors is limited to applications where the temperature is within a –55° to 150°C range. The measurement range of IC temperature sensors may be small compared to that of thermocouples and RTDs, but they have several advantages: they are small, accurate, and inexpensive, and are easy to interface with other devices such as amplifiers, regulators, DSPs, and microcontrollers.
IC temperature sensors continue to evolve, providing a varied array of functions, features, and interfaces. With the higher level of integration now feasible, digital IC temperature sensors can report both local and remote temperatures, monitor other system parameters, control fans, or warn when a specific temperature is exceeded.
There are two main types of IC temperature sensors?analog and digital?and several variations of each type. Analog sensors can produce a voltage or current proportional to temperature. Digital sensors may monitor local and/or remote temperatures, and can also include features such as voltage monitoring, fan control, and over- or under-limit alarms.
The earliest type of IC temperature sensor improved on the existing thermistor solutions whose resistance varied with temperature in a nonlinear fashion. Analog temperature sensors eliminated the need for additional linearization circuitry to correct for thermistor nonlinearity.
While newer digital output temperature sensors have supplanted analog output temperature sensors in many applications, analog temperature sensors continue to find a home in applications that do not require a digitized output. For example, the AD590 sensor is still a viable product in many applications more than 25 years after its release. Often used in remote applications as its high-impedance current makes it insensitive to voltage drops over long lines, it can be used in a wide variety of temperature-sensing applications not only because it operates up to 150°C, but also because of its wide +4 to +30-V operating voltage range.
Many applications require temperature data ultimately in a digital format, which can be achieved by feeding the output of an analog output temperature sensor into an A/D converter. With advances in IC production technology, however, it has become cost effective to integrate the A/D conversion?and indeed many other functions?onto the temperature sensor die, leading to cost, space, and power savings while simplifying the system designer's task.
Digital temperature sensors are similar to analog temperature sensors, but instead of outputting the data in current or voltage, it is converted into a digital format of 1's and 0's. Digital-output temperature sensors are therefore particularly useful when interfacing to a microcontroller. Such interfaces include single-wire PWM, two-wire I2C and SMBus, and three-/four-wire SPI protocols.
